Design And Control Strategy Of Transition Metal Catalysts For Electrochemical Nitrogen Reduction Reaction

As a critical feedstock,ammonia（NH₃）is important for agriculture and industry.The centuries-old industrial synthesis of NH₃via the Haber-Bosch mature technology is carried out with consumption of limited fossil energy and emission of large amounts of CO₂.Therefore,it is vital to develop a low-energy,green and substitutable process for the synthesis of ammonia.The electrochemical N₂reduction reaction（NRR）,driven by renewably generated electricity（solar,wind,etc.）and H₂O for NH₃synthesis,is now the subject of renewed interest.The attractiveness of this strategy lies on the potential not only to decarbonize the ammonia synthesis but also to disrupt the industrial model itself,enabling a decentralized production that circumvents large infrastructure investments.NRR is still plagued by three major issues.One is low NH₃Faradaic efficiencies（FE）because of severe competitive hydrogen reduction reaction（HER）catalyzed at similar potentials.The other is the unsatisfactory NH₃yield rate(YR_NH3).In addition,most of reported NRR electrocatalysts suffer from bottlenecks of rapid deactivation of electrodes.These problems restrict practical use and technological commercialization.The catalysts are the key component of reaction.As low cost,abundant and environmentally benign elements,transition metals offer a practical metal choice that possesses a suitable energy and symmetry with unoccupied nonbonding 3d orbitals being conducive to nitrogen adsorption and activation,in favour of nitrogen reduction reaction.This thesis focuses on transition-metal materials for NRR.Modulation of the structure of electrocatalysts can be achieved by tailoring several important aspects:heteroatom doping,surface active areas,surface vacancies,surface porosity and exposed crystal facets.The above catalyst design strategies have improved NH₃yield rate and FE,together with density functional theory（DFT）,providing a crucial catalyst design and theoretical basis for the industrial process of NRR.There are five subjects in this research as follows:1.ZIF-67 derived Co/NC composites enables efficient NRR by controlling carbonization temperature（400,500,600 and 700 ^oC）to adjust the composition and content of Co and N.The sample calcined at500 ^oC（Co/NC＿500）possesses a maximum YR_NH3of 5.1μg _NH3h^-1mg_cat^-1at-0.4 V（vs.RHE）and a FE of up to 10.1%at-0.1 V（vs.RHE）in 0.1 M KOH electrolyte.Co/NC＿500 with the preservation of a typical polyhedral shape typical of the ZIF-67 precursor shows large surface area,which benefits for the adsorption of N₂.A large number of single Co sites were observed in Co/NC＿500.The maximal active metal atoms utilization of Co single sites can manipulate the catalytic activity and selectivity,bringing in improved catalytic performance for the conversion of N₂to NH₃.The Co single atoms along with pyrrolic N are likely the major active sites.Moreover,this one-step carbonization method was simple and quick to prepare single atom catalyst.2.MoP₂nanosheets with adjustable thickness were prepared by ultrasonic exfoliation.After comparing the NRR and HER activities of Mo P₂nanosheets in different electrolytes（H₂SO₄,KOH,Na₂SO₄,Li₂SO₄,CH₃OH and CH₃COOLi-CH₃OH）,we found that CH₃OH and Li⁺in CH₃COOLi-CH₃OH electrolyte could inhibited HER and promoted NRR,respectively,which showed maximum YR_NH3of 8.22μg _NH3h^-1mg^-1_catand a FE of up to 20.62%.Comparison of the（111）with（020）facets in Mo P₂combined with DFT calculation suggested that（111）facets show stronger N₂adsorption,lower energy barrier of potential determining step and stronger interaction with Li⁺.Therefore,the（111）facets of Mo P₂played a major role in NRR reaction,and the mechanism follows an associative distal pathway.3.We judiciously design a well-defined porous core-shell MnO_x,comprising Mn₂O₃-Mn O（as the core）and Mn₃O₄（as the shell）.The thickness of the shell is tunable by controlling the reaction parameters.The unique composite featuring a high density of surface-exposed sites,pores,and oxygen vacancies is shown to efficiently optimize the electronic configuration through interfacial electron transfer in the heterostructure,which is beneficial for facilitating N₂adsorption and increasing the steady-state concentration of the reactants in the rate-controlling step of the NRR.Core-shell Mn O_xdelivers an impressive NH₃FE（23.8%）with a reasonable YR_NH3(22.4μg _NH3h^-1mg^-1_cat)at a cathodic voltage of-0.3 V（vs.RHE）in 0.1 M Na₂SO₄.The performance of Core-shell Mn O_xsurpasses that of most catalysts reported in the prior literature.Equally importantly,the electrocatalytic activity maintains good stability up to 60 h.4.Theoretical calculations revealed that Nb atoms delivered multifunctional enhancement toward the NRR when incorporated in（110）facets of Ti O₂:（1）Inducing electrons to promote the conductivity of Ti O₂,（2）activating the inert Ti sites for N₂adsorption,（3）suppressing the undesired competitive HER,（4）reducing the energy barrier of the potential-determining step,further facilitating NH₃formation.Accordingly,we successfully exposed the（110）facets of anatase Ti O₂and loaded Nb single atoms as a new NRR electrocatalyst.As a result,our Nb-Ti O₂（110）catalyst exhibits superior activity and selectivity for the NRR,which affords a YR_NH3of 21.27μg _NH3h^-1mg_cat^-1and NH₃FE of9.17%at-0.5 V（vs.RHE）in 0.1 M Na₂SO₄.The performance of Nb-Ti O₂（110）exceeds most previously reported Ti O₂-and Nb-based electrocatalysts for NRR at ambient conditions.Operando Raman spectroscopy was employed to monitor the reaction intermediates and products,further confirming the NRR activity of Nb-TiO₂（110）in accordance with the DFT results.